Quoted from the original DXF 12 Reference which is not available on the web:

Since the AutoCAD drawing database (.dwg file) is written in a
compact format that changes significantly as new features are added
to AutoCAD, we do not document its format and do not recommend that
you attempt to write programs to read it directly. To assist in
interchanging drawings between AutoCAD and other programs, a Drawing
Interchange file format (DXF) has been defined. All implementations
of AutoCAD accept this format and are able to convert it to and from
their internal drawing file representation.

The Drawing class manages all entities and tables related to a
DXF drawing. You can read DXF drawings from file-system or from a text-stream
and you can also write the drawing to file-system or to a text-stream.